This invention relates to magnetoresistive transducers having a permanent magnet bias, which transducers are particularly useful for reproduction of signals recorded on narrow tracks of a magnetic medium.
Magnetoresistive transducers are known in the art of reproducing information signals from magnetic tape, disc, card, sheet or other magnetic medium. These transducers comprise a magnetoresistive element, further referred to as MR element, in the form of an elongated narrow strip. The strip is made of a low anisotropy ferromagnetic material which has an easy axis of magnetization oriented usually along a length thereof. In accordance with the well known magnetoresistance effect, the strip exhibits a change in resistivity which is caused by a change in the direction of magnetization thereof, in response to a changing magnetic field externally applied thereto. Because of the well known nonlinearity of the response curve, it is necessary to apply a magnetic bias field to the MR element to obtain operation within a linear portion of the curve. The magnitude of the output signal from the MR element is proportional to its length which, in turn, corresponds to the recording track width. Therefore linearizing the response and maximizing the sensitivity of the MR element are particularly important in connection with reproduction from high storage density magnetic medium where the recording track width is reduced to a minimum.
Various known methods of biasing MR elements include utilizing an external bias magnet, providing conductive strips for carrying sense current at 45 degrees to the longitudinal axis of the element, and providing an internal biasing permanent magnet layer of high coercivity magnetic film integrally within the magnetoresistive transducer structure. Use of the integral magnetic layer has apparent advantages over other biasing methods as follows. It simplifies the structure and thus the manufacturing process of the MR transducer assembly. Furthermore, it is an improvement over the use of external biasing magnets which do not allow the use of magnetic shields. Such shields are interposed between the magnet and MR element and thus they would interfere with the bias field.
Internally biased prior art magnetoresistive transducers have a high coercivity permanent magnet biasing layer arranged within the transducer structure in the shape of an elongated strip matching the shape of the MR element superposed therewith. To obtain a linear change in resistivity of the MR element with a changing magnetic field applied thereto, it has been necessary to magnetize the permanent magnet layer at approximately 45 degrees with reference to its length. The direction of the length corresponds to the easy axis of magnetization of the MR element. However, due to the well known properties of these longitudinally shaped permanent magnets, which generally have an aspect ratio from 6:1 to 20:1, a relatively large bias field component has been provided in the direction of the depth of the magnet while a relatively small bias field component has been provided in the direction of its length. Consequently, the bias field component applied to the underlying MR element in the easy axis direction has a considerably smaller magnitude when compared to a component directed perpendicularly thereto. As an example, an estimated magnitude of the field in easy axis direction is 15 Oe while a corresponding magnitude in the hard axis direction is 100 Oe, approximately.
Under these conditions the internal bias magnet cannot provide a sufficiently strong and stable magnetization of the MR element at 45 degrees and deterioration in the linearity of signal response results. In addition, the thusly biased MR element is very sensitive to external stray fields.
Laboratory tests performed on a prior art magnetoresistive transducer having an elongated biasing permanent magnet strip indicate strong sensitivity to externally applied D.C. stray fields. As an example, such an MR element has been excited by an A.C. signal field of 40 Hz, 2 Oe and a D.C. stray field in the range between .+-.10 Oe was applied thereto. Typically the MR element exhibited a 10 dB decrease in sensitivity in response to the stray field. In addition, the MR element exhibited a large second harmonic distortion, which is comparable to the signal level, in response to relatively small stray fields of less than 2 Oe.